When you create a SharePoint calendar, by default, you can’t add attendees to events:

To add the attendees part in this windows, follows these steps:

  • Go to List Settings in the Calendar tab.
  • In the Content Types part, click on Event.
  • In the Columns part, click on Add from existing site or list columns.
  • Select Attendees and click on Add >.

Now, if you go back to your calendar, and if you create/edit an event, you will be able to add attendees to this item: